The direct oxidation of ethylene to ethylene oxide by molecular oxygen is well-known and is, in fact, the method used currently for commercial production of ethylene oxide. The typical catalyst for such purpose contains metallic or ionic silver, optionally modified with various promoters and activators. Most such catalysts contain a porous, inert support or carrier such as alpha alumina upon which the silver and promoters are deposited. A review of the direct oxidation of ethylene in the presence of supported silver catalysts is provided by Sachtler et al. in Catalyst Reviews: Science and Engineering, 23 (1&2), 127-149 (1981).
It is also well-known, however, that the catalysts and reaction conditions which are best suited for ethylene oxide production do not give comparable results in the direct oxidation of higher olefins such as propylene. The discovery of processes capable of providing propylene oxide by vapor phase direct oxidation in higher yields than are presently attainable thus would be most desirable.
The benefits of including carbon dioxide in the feedstream of a direct ethylene epoxidation process catalyzed by a supported silver catalyst have long been generally recognized by workers in the field. For example, Lefort, in U.S. Pat. No. 1,998,878, states that carbon dioxide may be introduced into the reactor to limit the rate of complete oxidation of ethylene to carbon dioxide. U.S. Pat. No.2,615,900 discloses a process for producing ethylene oxide in which carbon dioxide gas is added to the feed gases to act as a "depressant" or "ant-catalytic material", thereby improving ethylene oxide selectivity. See also U.S. Pat. No. 4,007,135, which indicates that carbon dioxide may be used to raise the selectivity of the reaction.
Supported silver catalysts have traditionally been prepared by impregnating a support with a soluble silver compound and then converting the silver compound to metallic silver by "reduction" or calcining the impregnated support at an elevated temperature. U.S. Pat. No. 3,943,069 teaches such a process wherein calcination is performed in a gaseous atmosphere containing at least 1% by volume carbon dioxide. Catalysts prepared in this manner are said to have improved mechanical properties as well as high activity and selectivity in the oxidation of ethylene to ethylene oxide. Decomposition of a catalyst precursor containing a silver amine nitrate complex in an atmosphere containing carbon dioxide is suggested in U.S. Pat. No. 4,212,772. Related catalyst preparation methods employing carbon dioxide during a decomposition step are proposed in U.S. Pat. Nos. 4,066,575 and 4,400,308.
Silver catalysts supported on alkaline metal carbonates and promoted with both potassium and molybdenum compounds have been proposed for the direct conversion of propylene to propylene oxide using molecular oxygen (see U.S. Pat. No. 5,625,084). According to this patent, the inclusion of carbon dioxide in the feedstream improves propylene oxide selectivity. There is no suggestion, however, that conditioning the catalyst with carbon dioxide could lead to lasting enhancement of catalytic performance when a carbon dioxide-free feedstream is subsequently employed.